The present invention refers to a method and apparatus for electrostatic coating of workpieces with powdered material, in particular to a method and apparatus of a type by which the workpieces are advanced through several successively arranged spray guns supplied with a powder-air mixture, with sprayed excessive powder being used for renewed spraying through recycling.
In general, in conventional methods and apparatuses of this type the workpieces are advanced, for example suspended from a chain, through a spray cabin which houses several successive spray guns so that subsequent coatings are applied upon the workpiece which, after passing the last spray gun, has a powder coating of desired thickness. During such treatment, a considerable amount of excess powder i.e. sprayed powder which does not adhere to the workpiece is obtained. This amount of excess powder is immediately discharged from the cabin "continuously" and/or during a paint change and collected in containers for further use (recycling). Although such recycling does reduce powder losses, there are still drawbacks. Even when thoroughly cleansing the cabin during each change of paint, it is still unavoidable, in particular during frequent paint change, that powder particles of paint from a previous coating process are blended into the recycled powder so that a renewed use of the recycled powder results in stains on the surface of the coated workpiece and thus to a diminished surface quality (finish). This is true even if the fraction of foreign paint particles is slight. Proposals to add fresh powder to the recycled excess powder did not resolve this problem because--as set forth above--even smallest amounts of foreign paint particles will mar the finish.
Practice has further shown that during spraying of recycled excess powder, especially when the latter is recycled for several times, those areas of the workpiece which by nature are difficult to coat, such as areas which extend in the electrostatic "field shadow", the coating results remain unsatisfactory. A reason for that could be the enrichment of fiiller particles in the recycled and multiply-used powder, on the one hand, and the high kinetic energy of the paint particles of the powder-air spray stream which exits the spray gun and contains a substantial amount of air.